Cu nanoparticles enable plasmonic-improved silicon photovoltaic devices

This work examines the effect of copper nanoparticles (Cu NPs) on the photocurrent efficiency of silicon photovoltaic (Si PV) devices. An optimized synthesis of stable Cu NPs is reported together with a procedure for their immobilization on the Si PV surface. A comprehensive analysis of the photocurrent and power dependence of the Cu NPs surface coverage and size is presented. A decrease in photoconversion was observed for wavelengths shorter than similar to 500 nm, due to the Cu interband absorption. In the low surface coverage limit, where the level of aggregation was found to be low, the surface plasmon resonance absorption dominates leading to a modest effect on the photocurrent response. As the number of aggregates increased with the surface coverage, the photocurrent efficiency also increased, and a maximum enhancement power conversion of 16% was found for a 54 +/- 6 NPs per mu m(2) PV cell. This enhancement was attributed to SPR light scattering and trapping into the Si PV device. Higher surface coverage yielded numerous aggregates which acted as a bulk coating and caused a decrease in both photocurrent and power measurements.

Photoconduction action spectra of regio-regular poly(3-hexylthiopene):TiO2 diodes

The development of polymer-based photovoltaic devices brings the promise of low-cost and lightweight solar energy conversion systems. This technology requires new materials and device architectures with enhanced efficiency and lifetime, which depends on the understanding of charge-transport mechanisms. Organic films combined with electronegative nanoparticles may form systems with efficient dissociation of the photogenerated excitons, thus increasing the number of carriers to be collected by the electrodes. In this paper we investigate the steady-state photoconductive action spectra of devices formed by a bilayer of regio-regular poly(3-hexylthiophene) (RRP3HT) and TiO2 sandwiched between ITO and aluminum electrodes (ITO/TiO2:RRP3HT/Al). Photocurrents were measured for distinct bias voltages with illumination from either side of the device. Heterojunction structures were prepared by spin coating a RRP3HT film on an already deposited TiO2 layer on ITO. Symbatic and antibatic curves were obtained and a model for photocurrent action spectra was able to fit the symbatic responses. The quantum yield increased with the electric field, indicating that exciton dissociation is a field-assisted process as in an Onsager mechanism. Furthermore, the quantum yield was significantly higher when illumination was carried out through the ITO electrode onto which the TiO2 layer was deposited, as the highly electronegative TiO2 nanoparticles were efficient in exciton dissociation.

Betrachtung von chemischen Reaktionen und mesoskopischen Strukturen auf Oberflächen mittels spektroskopischer Photoelektronenmikroskopie

In dieser Arbeit wurden Untersuchungen mit verschiedenenMethoden derMikrospektroskopie unter Verwendung einesPhotoemissions-Elektronenmikroskops(PEEM) durchgeführt. Es wurde ein Energiefilterfür den Einsatzmit dem PEEM aufgebaut und getestet. Dieses Instrument wurdezusammen miteinem Multilayer-Monochromator am Synchrotron eingesetzt.Der Energiefilterwurde unter anderem für Photoelektronenspektroskopie anAu_xCs_1-xund Pb/Si-Schichten, sowie zur Charakterisierung desMonochromators eingesetzt.
Die Transmissionsfunktion eines abbildendenGegenfeldanalysators fürdas PEEM wurde berechnet und mit Messungen verglichen. InÜbereinstimmungmit der Berechnung zeigt sich ein charakteristischesasymmetrisches Profilder mit dem Analysator gemessenen Photoelektronenpeaks. Eswurde eine Verbesserungder Ortsauflösung durch energieselektive Abbildungnachgewiesen.
Mit dem PEEM wurde erstmals Mehr-Photonen Photoemissionzur Abbildungausgenutzt. Es wurde ein Verfahren gezeigt, mit dem sich dieOrdnung dernichtlinearen Photoemission aus zwei mit unterschiedlicherLaserleistungaufgenommenen Bildern lateral abbilden läßt. DiePolarisationsabhängigkeitder Photoemission von glatten Probenstellen läßtsich mit einercos⁴-Funktion anpassen, wie es fürZwei-Photonen Photoemissionzu erwarten ist. Alle Proben wiesen Zentren erhöhterPhotoemissionsintensitätauf, welche durch Anregung lokalisierter Plasmonen auf derOberflächean kleinen Partikeln entstehen. An diesen erstmalig sichtbargemachten'Hot Spots' zeigt sich eine charakteristischePolarisationsabhängigkeitder Photoemission mit bevorzugter Anregung vonPhotoelektronen bei s-polarisiertemLicht.

Photoemissions-Elektronenmikroskopie an Adsorbatsystemen

Die Photoemissions-Elektronenmikroskopie ist eine hervorragend geeignete Methode zur Untersuchung dynamischer Vorgänge auf realen polykristallinen Oberflächen im sub-μm Bereich. Bei der Anwendung auf Adsorbatsysteme lassen sich geringe Bedeckungsunterschiede, sowie Adsorbatstrukturen und -phasen unterscheiden. Die Methode erlaubt dabei ein breites Anwendungsspektrum über weite Temperaturbereiche und Systeme unterschiedlichster Bindungsenergie. Bei der Chemisorption von Sauerstoff auf polykristallinen Metallen wird unterschiedliches Aufwachsverhalten in den Helligkeitswerten im Mikroskopbild widergespiegelt. Bei Kupferproben zeigen Oberflächen mit unterschiedlicher kristalliner Richtung aufgrund der Symmetrie des fcc-Gitters ein ähnliches Verhalten. Das hexagonale Gitter des Titans zeigt dagegen große Unterschiede im Adsorptionsverhalten in Abhängigkeit der kristallinen Richtung. Diese Unterschiede konnten auf verschiedene Haftkoeffizienten und Oxidationsstufen der Metalle zurückgeführt werden. In einem Modell zur Photostromanalyse konnte beim Kupfer der Übergang von verschiedenen Überstrukturen bei wachsender Bedeckung gezeigt und die Übergänge ermittelt werden.. Auf den Titanoberflächen wurde so das Wachstum der Oxide TiO, TiO₂ und Ti₂O₃ unterschieden und die Übergänge des unterschiedlichen Wachstums ermittelt. Bei der thermischen Desorption der Schichten konnten unterschiedliche Haftkoeffizient auf einzelnen Kristalliten qualitativ gezeigt werden. Diese erstmalig eingesetzte Analysemethode weist Ähnlichkeiten zur Thermo-Desorptions-Spektroskopie (TDS) auf, zeigt jedoch ortsaufgelöst lokale Unterschiede auf polykristallinen Oberflächen. Bei thermisch gestützten Oberflächenreaktionen ließen sich die Reaktionskeime deutlich identifizieren und mit einer Grauwertanalyse konnte die Oxidation der karbidischen Lagen zu Kohlenmonoxid und die Metalloxidation unterschieden werden. Dabei konnte gezeigt werden, daß die Reaktionskeime nur an Plattengrenzen auftreten, nicht jedoch auf der Oberfläche. Durch die Aufrauhung der Plattengrenzen mit zunehmender Reaktionsdauer nimmt die Zahl der Reaktionskeime kontinuierlich zu, die laterale Ausdehnung der Einzelreaktionen bleibt aber konstant. Bei der Physisorption von Xenon auf Graphit wurde erstmals für die Photoemissionsmikroskopie die resonanten Anregung ausgenutzt. Die verschiedenen Phasen des Adsorbats können dabei deutlich unterschieden werden; bei niedrigen Temperaturen (40K) findet ein gleichmäßiges Wachstum auf der gesamten Oberfläche statt, bei höheren Temperaturen von 60-65K ist dagegen ein Inselwachstum in verschiedenen Phasen zu beobachten. Die zeitliche Entwicklung des Wachstums, die örtliche Lage der Phasen und die Phasenübergänge (gas, fest inkommensurabel, fest kommensurabel) konnten bestimmt werden. Bei der Desorption der Schichten konnten die einzelnen Phasen ebenfalls getrennt werden und das unterschiedliche Desorptionsverhalten sowie die Phasenübergänge selber verifiziert werden.

Electronic energy transfer processes and charge carrier transport in pi-conjugated polymers

Conjugated polymers have attracted tremendous academical and industrial research interest over the past decades due to the appealing advantages that organic / polymeric materials offer for electronic applications and devices such as organic light emitting diodes (OLED), organic field effect transistors (OFET), organic solar cells (OSC), photodiodes and plastic lasers. The optimization of organic materials for applications in optoelectronic devices requires detailed knowledge of their photophysical properties, for instance energy levels of excited singlet and triplet states, excited state decay mechanisms and charge carrier mobilities. In the present work a variety of different conjugated (co)polymers, mainly polyspirobifluorene- and polyfluorene-type materials, was investigated using time-resolved photoluminescence spectroscopy in the picosecond to second time domain to study their elementary photophysical properties and to get a deeper insight into structure-property relationships. The experiments cover fluorescence spectroscopy using Streak Camera techniques as well as time-delayed gated detection techniques for the investigation of delayed fluorescence and phosphorescence. All measurements were performed on the solid state, i.e. thin polymer films and on diluted solutions. Starting from the elementary photophysical properties of conjugated polymers the experiments were extended to studies of singlet and triplet energy transfer processes in polymer blends, polymer-triplet emitter blends and copolymers. The phenomenon of photonenergy upconversion was investigated in blue light-emitting polymer matrices doped with metallated porphyrin derivatives supposing an bimolecular annihilation upconversion mechanism which could be experimentally verified on a series of copolymers. This mechanism allows for more efficient photonenergy upconversion than previously reported for polyfluorene derivatives. In addition to the above described spectroscopical experiments, amplified spontaneous emission (ASE) in thin film polymer waveguides was studied employing a fully-arylated poly(indenofluorene) as the gain medium. It was found that the material exhibits a very low threshold value for amplification of blue light combined with an excellent oxidative stability, which makes it interesting as active material for organic solid state lasers. Apart from spectroscopical experiments, transient photocurrent measurements on conjugated polymers were performed as well to elucidate the charge carrier mobility in the solid state, which is an important material parameter for device applications. A modified time-of-flight (TOF) technique using a charge carrier generation layer allowed to study hole transport in a series of spirobifluorene copolymers to unravel the structure-mobility relationship by comparison with the homopolymer. Not only the charge carrier mobility could be determined for the series of polymers but also field- and temperature-dependent measurements analyzed in the framework of the Gaussian disorder model showed that results coincide very well with the predictions of the model. Thus, the validity of the disorder concept for charge carrier transport in amorphous glassy materials could be verified for the investigated series of copolymers.

Polimeri tiofenici funzionali e loro applicazioni nel fotovoltaico

Recentemente, sempre più attenzione è stata rivolta all' utilizzo di coloranti organici come assorbitori di luce per la preparazione di strati fotoattivi in celle solari organiche (OPV). I coloranti organici presentano un'elevata abilità nella cattura della luce solare grazie all'elevato coefficiente di estinzione molare e buone proprietà fotofisiche. Per questi motivi sono eccellenti candidati per l'incremento della conversione fotoelettrica in OPV. In questa tesi viene descritta una nuova strategia per l'incorporazione di derivati porfirinici in catena laterale a copolimeri tiofenici. Gli studi svolti hanno dimostrato che poli(3-bromoesil)tiofene può essere variamente funzionalizzato con idrossitetrafenilporfirina (TPPOH), per l'ottenimento di copolimeri utilizzabili come materiali p-donatori nella realizzazione di OPV. I copolimeri poli[3-(6-bromoesil)tiofene-co-(3-[5-(4-fenossi)-10,15,20-trifenilporfirinil]esil tiofene] P[T6Br-co-T6TPP] contenenti differenti quantità di porfirina, sono stati sintetizzati sia con metodi non regiospecifici che regiospecifici, con lo scopo di confrontarene le proprietà e di verificare se la strutture macromolecolare che presenta una regiochimica di sostituzione sempre uguale, promuove o meno il trasporto della carica elettrica, migliorando di conseguenza l'efficienza. E' stato inoltre effettuato un ulteriore confronto tra questi derivati e derivati simili P[T6H-co-T6TPP] che non contengono l'atomo di bromo in catena laterale con lo scopo di verificare se l'assenza del gruppo reattivo, migliora o meno la stabilità termica e chimica dei film polimerici, agendo favorevolmete sulle performance dei dispositivi fotovoltaici. Tutti i copolimeri sono stati caratterizzati con differenti tecniche: spettroscopia NMR, FT-IR e UV-Vis, analisi termiche DSC e TGA, e GPC. Le celle solari Bulk Heterojunction, preparate utilizzando PCBM come materiale elettron-accettore e i copolimeri come materilai elettron-donatori, sono state testate utilizzando un multimetro Keithley e il Solar Simulator.

Electrical spectroscopy of rubrene bulk and thin film crystals

One of the most diffused electronic device is the field effect transistor (FET), contained in number of billions in each electronic device. Organic optoelectronics is an emerging field that exploits the unique properties of conjugated organic materials to develop new applications that require a combination of performance, low cost and processability. Organic single crystals are the material with best performances and purity among the variety of different form of organic semiconductors. This thesis is focused on electrical and optical characterization of Rubrene single crystal bulk and thin films. Rubrene bulk is well known but for the first time we studied thin films. The first Current-voltage characterization has been performed for the first time on three Rubrene thin films with three different thickness to extract the charge carriers mobility and to assess its crystalline structure. As results we see that mobility increase with thickness. Field effect transistor based on Rubrene thin films on $SiO_2$ have been characterize by current-voltage (I-V) analyses (at several temperatures) and reveals a hopping conduction. Hopping behavior probably is due to the lattice mismatch with the substrate or intrinsic defectivity of the thin films. To understand effects of contact resistance we tested thin films with the Transmission Line Method (TLM) method. The TLM method revealeds that contact resistance is negligible but evidenced a Schottky behavior in a limited but well determined range of T. To avoid this effect we carried out annealing treatment after the electrode evaporation iswe performed a compete I-V characterization as a function of in temperature to extract the electronic density of states (DOS) distribution through the Space Charge Limited Current (SCLC) method. The results show a DOS with an exponential trenddistribution, as expected. The measured mobility of thin films is about 0.1cm^2/Vs and it increases with the film thickness. Further studies are necessary to investigate the reason and improve performances. From photocurrent spectrum we calculated an Eg of about 2.2eV and both thin films and bulk have a good crystal order. Further measurement are necessary to solve some open problems

Transport Properties and Novel Sensing Applications of Organic Semiconducting Crystals

The present thesis is focused on the study of Organic Semiconducting Single Crystals (OSSCs) and crystalline thin films. In particular solution-grown OSSC, e.g. 4-hdroxycyanobenzene (4HCB) have been characterized in view of their applications as novel sensors of X-rays, gamma-rays, alpha particles radiations and chemical sensors. In the field of ionizing radiation detection, organic semiconductors have been proposed so far mainly as indirect detectors, i.e. as scintillators or as photodiodes. I first study the performance of 4HCB single crystals as direct X-ray detector i.e. the direct photon conversion into an electrical signal, assessing that they can operate at room temperature and in atmosphere, showing a stable and linear response with increasing dose rate. A dedicated study of the collecting electrodes geometry, crystal thickness and interaction volume allowed us to maximize the charge collection efficiency and sensitivity, thus assessing how OSSCs perform at low operating voltages and offer a great potential in the development of novel ionizing radiation sensors. To better understand the processes generating the observed X-ray signal, a comparative study is presented on OSSCs based on several small-molecules: 1,5-dinitronaphthalene (DNN), 1,8-naphthaleneimide (NTI), Rubrene and TIPS-pentacene. In addition, the proof of principle of gamma-rays and alpha particles has been assessed for 4HCB single crystals. I have also carried out an investigation of the electrical response of OSSCs exposed to vapour of volatile molecules, polar and non-polar. The last chapter deals with rubrene, the highest performing molecular crystals for electronic applications. We present an investigation on high quality, millimeter-sized, crystalline thin films (10 – 100 nm thick) realized by exploiting organic molecular beam epitaxy on water-soluble substrates. Space-Charge-Limited Current (SCLC) and photocurrent spectroscopy measurements have been carried out. A thin film transistor was fabricated onto a Cytop® dielectric layer. The FET mobility exceeding 2 cm2/Vs, definitely assess the quality of RUB films.

Charge generation and recombination in hybrid organic,inorganic solar cells

This thesis deals with the investigation of exciton and charge dynamics in hybrid solar cells by time-resolved optical spectroscopy. Quasi-steady-state and transient absorption spectroscopy, as well as time-resolved photoluminescence spectroscopy, were employed to study charge generation and recombination in solid-state organic dye-sensitized solar cells, where the commonly used liquid electrolyte is replaced by an organic solid hole transporter, namely 2,2′7,7′-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-MeOTAD), and polymer-metal oxide bulk heterojunction solar cells, where the commonly used fullerene acceptor [6,6]-phenyl C61 butyric acid methyl ester (PCBM) is replaced by zinc oxide (ZnO) nanoparticles. By correlating the spectroscopic results with the photovoltaic performance, efficiency-limiting processes and processes leading to photocurrent generation in the investigated systems are revealed. rnIt is shown that the charge generation from several all-organic donor-π-bridge-acceptor dyes, specifically perylene monoimide derivatives, employed in solid-state dye-sensitized solar cells, is strongly dependent on the presence of a commonly used additive lithium bis(trifluoromethanesulphonyl)imide salt (Li-TFSI) at the interface. rnMoreover, it is shown that charges can not only be generated by electron injection from the excited dye into the TiO2 acceptor and subsequent regeneration of the dye cation by the hole transporter, but also by an alternative mechanism, called preceding hole transfer (or reductive quenching). Here, the excited dye is first reduced by the hole transporter and the thereby formed anion subsequently injects an electron into the titania. This additional charge generation process, which is only possible for solid hole transporters, helps to overcome injection problems. rnHowever, a severe disadvantage of solid-state dye-sensitized solar cells is re-vealed by monitoring the transient Stark effect on dye molecules at the inter-face induced by the electric field between electrons and holes. The attraction between the negative image charge present in TiO2, which is induced by the positive charge carrier in the hole transporter due to the dielectric contrast between the organic spiro-MeOTAD and inorganic titania, is sufficient to at-tract the hole back to the interface, thereby increasing recombination and suppressing the extraction of free charges.rnBy investigating the effect of different dye structures and physical properties on charge generation and recombination, design rules and guidelines for the further advancement of solid-state dye-sensitized solar cells are proposed.rnFinally, a spectroscopic study on polymer:ZnO bulk heterojunction hybrid solar cells, employing different surfactants attached to the metal oxide nanoparticles, was performed to understand the effect of surfactants upon photovoltaic behavior. By applying a parallel pool analysis on the transient absorption data, it is shown that suppressing fast recombination while simultaneously maintaining the exciton splitting efficiency by the right choice of surfactants leads to better photovoltaic performances. Suppressing the fast recombination completely, whilst maintaining the exciton splitting, could lead to a doubling of the power conversion efficiency of this type of solar cell.

Unraveling efficiency-limiting processes in organic solar cells by ultrafast spectroscopy - impact of chemical structure and morphology on photophysics and efficiency

Diese Arbeit widmet sich der Untersuchung der photophysikalischen Prozesse, die in Mischungen von Elektronendonoren mit Elektronenakzeptoren zur Anwendung in organischen Solarzellen auftreten. Als Elektronendonoren werden das Copolymer PBDTTT-C, das aus Benzodithiophen- und Thienothiophene-Einheiten besteht, und das kleine Molekül p-DTS(FBTTh2)2, welches Silizium-überbrücktes Dithiophen, sowie fluoriertes Benzothiadiazol und Dithiophen beinhaltet, verwendet. Als Elektronenakzeptor finden ein planares 3,4:9,10-Perylentetracarbonsäurediimid-(PDI)-Derivat und verschiedene Fullerenderivate Anwendung. PDI-Derivate gelten als vielversprechende Alternativen zu Fullerenen aufgrund der durch chemische Synthese abstimmbaren strukturellen, optischen und elektronischen Eigenschaften. Das gewichtigste Argument für PDI-Derivate ist deren Absorption im sichtbaren Bereich des Sonnenspektrums was den Photostrom verbessern kann. Fulleren-basierte Mischungen übertreffen jedoch für gewöhnlich die Effizienz von Donor-PDI-Mischungen.rnUm den Nachteil der PDI-basierten Mischungen im Vergleich zu den entsprechenden Fulleren-basierten Mischungen zu identifizieren, werden die verschiedenen Donor-Akzeptor-Kombinationen auf ihre optischen, elektronischen und strukturellen Eigenschaften untersucht. Zeitaufgelöste Spektroskopie, vor allem transiente Absorptionsspektroskopie (TA), wird zur Analyse der Ladungsgeneration angewendet und der Vergleich der Donor-PDI Mischfilme mit den Donor-Fulleren Mischfilmen zeigt, dass die Bildung von Ladungstransferzuständen einen der Hauptverlustkanäle darstellt.rnWeiterhin werden Mischungen aus PBDTTT-C und [6,6]-Phenyl-C61-buttersäuremethylesther (PC61BM) mittels TA-Spektroskopie auf einer Zeitskala von ps bis µs untersucht und es kann gezeigt werden, dass der Triplettzustand des Polymers über die nicht-geminale Rekombination freier Ladungen auf einer sub-ns Zeitskala bevölkert wird. Hochentwickelte Methoden zur Datenanalyse, wie multivariate curve resolution (MCR), werden angewendet um überlagernde Datensignale zu trennen. Zusätzlich kann die Regeneration von Ladungsträgern durch Triplett-Triplett-Annihilation auf einer ns-µs Zeitskala gezeigt werden. Darüber hinaus wird der Einfluss des Lösungsmitteladditivs 1,8-Diiodooctan (DIO) auf die Leistungsfähigkeit von p-DTS(FBTTh2)2:PDI Solarzellen untersucht. Die Erkenntnisse von morphologischen und photophysikalischen Experimenten werden kombiniert, um die strukturellen Eigenschaften und die Photophysik mit den relevanten Kenngrößen des Bauteils in Verbindung zu setzen. Zeitaufgelöste Photolumineszenzmessungen (time-resolved photoluminescence, TRPL) zeigen, dass der Einsatz von DIO zu einer geringeren Reduzierung der Photolumineszenz führt, was auf eine größere Phasentrennung zurückgeführt werden kann. Außerdem kann mittels TA Spektroskopie gezeigt werden, dass die Verwendung von DIO zu einer verbesserten Kristallinität der aktiven Schicht führt und die Generation freier Ladungen fördert. Zur genauen Analyse des Signalzerfalls wird ein Modell angewendet, das den gleichzeitigen Zerfall gebundener CT-Zustände und freier Ladungen berücksichtigt und optimierte Donor-Akzeptor-Mischungen zeigen einen größeren Anteil an nicht-geminaler Rekombination freier Ladungsträger.rnIn einer weiteren Fallstudie wird der Einfluss des Fullerenderivats, namentlich IC60BA und PC71BM, auf die Leistungsfähigkeit und Photophysik der Solarzellen untersucht. Eine Kombination aus einer Untersuchung der Struktur des Dünnfilms sowie zeitaufgelöster Spektroskopie ergibt, dass Mischungen, die ICBA als Elektronenakzeptor verwenden, eine schlechtere Trennung von Ladungstransferzuständen zeigen und unter einer stärkeren geminalen Rekombination im Vergleich zu PCBM-basierten Mischungen leiden. Dies kann auf die kleinere Triebkraft zur Ladungstrennung sowie auf die höhere Unordnung der ICBA-basierten Mischungen, die die Ladungstrennung hemmen, zurückgeführt werden. Außerdem wird der Einfluss reiner Fullerendomänen auf die Funktionsfähigkeit organischer Solarzellen, die aus Mischungen des Thienothienophen-basierenden Polymers pBTTT-C14 und PC61BM bestehen, untersucht. Aus diesem Grund wird die Photophysik von Filmen mit einem Donor-Akzeptor-Mischungsverhältnis von 1:1 sowie 1:4 verglichen. Während 1:1-Mischungen lediglich eine co-kristalline Phase, in der Fullerene zwischen den Seitenketten von pBTTT interkalieren, zeigen, resultiert der Überschuss an Fulleren in den 1:4-Proben in der Ausbildung reiner Fullerendomänen zusätzlich zu der co kristallinen Phase. Transiente Absorptionsspektroskopie verdeutlicht, dass Ladungstransferzustände in 1:1-Mischungen hauptsächlich über geminale Rekombination zerfallen, während in 1:4 Mischungen ein beträchtlicher Anteil an Ladungen ihre wechselseitige Coulombanziehung überwinden und freie Ladungsträger bilden kann, die schließlich nicht-geminal rekombinieren.

Functional ZnO nanostructures for electronic and energy applications

ZnO has proven to be a multifunctional material with important nanotechnological applications. ZnO nanostructures can be grown in various forms such as nanowires, nanorods, nanobelts, nanocombs etc. In this work, ZnO nanostructures are grown in a double quartz tube configuration thermal Chemical Vapor Deposition (CVD) system. We focus on functionalized ZnO Nanostructures by controlling their structures and tuning their properties for various applications. The following topics have been investigated: 1. We have fabricated various ZnO nanostructures using a thermal CVD technique. The growth parameters were optimized and studied for different nanostructures. 2. We have studied the application of ZnO nanowires (ZnONWs) for field effect transistors (FETs). Unintentional n-type conductivity was observed in our FETs based on as-grown ZnO NWs. We have then shown for the first time that controlled incorporation of hydrogen into ZnO NWs can introduce p-type characters to the nanowires. We further found that the n-type behaviors remained, leading to the ambipolar behaviors of hydrogen incorporated ZnO NWs. Importantly, the detected p- and n- type behaviors are stable for longer than two years when devices were kept in ambient conditions. All these can be explained by an ab initio model of Zn vacancy-Hydrogen complexes, which can serve as the donor, acceptors, or green photoluminescence quencher, depend on the number of hydrogen atoms involved. 3. Next ZnONWs were tested for electron field emission. We focus on reducing the threshold field (Eth) of field emission from non-aligned ZnO NWs. As encouraged by our results on enhancing the conductivity of ZnO NWs by hydrogen annealing described in Chapter 3, we have studied the effect of hydrogen annealing for improving field emission behavior of our ZnO NWs. We found that optimally annealed ZnO NWs offered much lower threshold electric field and improved emission stability. We also studied field emission from ZnO NWs at moderate vacuum levels. We found that there exists a minimum Eth as we scale the threshold field with pressure. This behavior is explained by referring to Paschen’s law. 4. We have studied the application of ZnO nanostructures for solar energy harvesting. First, as-grown and (CdSe) ZnS QDs decorated ZnO NBs and ZnONWs were tested for photocurrent generation. All these nanostructures offered fast response time to solar radiation. The decoration of QDs decreases the stable current level produced by ZnONWs but increases that generated by NBs. It is possible that NBs offer more stable surfaces for the attachment of QDs. In addition, our results suggests that performance degradation of solar cells made by growing ZnO NWs on ITO is due to the increase in resistance of ITO after the high temperature growth process. Hydrogen annealing also improve the efficiency of the solar cells by decreasing the resistance of ITO. Due to the issues on ITO, we use Ni foil as the growth substrates. Performance of solar cells made by growing ZnO NWs on Ni foils degraded after Hydrogen annealing at both low (300 °C) and high (600 °C) temperatures since annealing passivates native defects in ZnONWs and thus reduce the absorption of visible spectra from our solar simulator. Decoration of QDs improves the efficiency of such solar cells by increasing absorption of light in the visible region. Using a better electrolyte than phosphate buffer solution (PBS) such as KI also improves the solar cell efficiency. 5. Finally, we have attempted p-type doping of ZnO NWs using various growth precursors including phosphorus pentoxide, sodium fluoride, and zinc fluoride. We have also attempted to create p-type carriers via introducing interstitial fluorine by annealing ZnO nanostructures in diluted fluorine gas. In brief, we are unable to reproduce the growth of reported p-type ZnO nanostructures. However; we have identified the window of temperature and duration of post-growth annealing of ZnO NWs in dilute fluorine gas which leads to suppression of native defects. This is the first experimental effort on post-growth annealing of ZnO NWs in dilute fluorine gas although this has been suggested by a recent theory for creating p-type semiconductors. In our experiments the defect band peak due to native defects is found to decrease by annealing at 300 °C for 10 – 30 minutes. One of the major future works will be to determine the type of charge carriers in our annealed ZnONWs.

Boron nitride nanotubes : synthesis, characterization, functionalization, and potential applications

Boron nitride nanotubes (BNNTs) are structurally similar to carbon nanotubes (CNTs), but exhibit completely different physical and chemical properties. Thus, BNNTs with various interesting properties may be complementary to CNTs and provide an alternative perspective to be useful in different applications. However, synthesis of high quality of BNNTs is still challenging. Hence, the major goals of this research work focus on the fundamental study of synthesis, characterizations, functionalization, and explorations of potential applications. In this work, we have established a new growth vapor trapping (GVT) approach to produce high quality and quantity BNNTs on a Si substrate, by using a conventional tube furnace. This chemical vapor deposition (CVD) approach was conducted at a growth temperature of 1200 °C. As compared to other known approaches, our GVT technique is much simpler in experimental setup and requires relatively lower growth temperatures. The as-grown BNNTs are fully characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), Energy Filtered Mapping, Raman spectroscopy, Fourier Transform Infra Red spectroscopy (FTIR), UV-Visible (UV-vis) absorption spectroscopy, etc. Following this success, the growth of BNNTs is now as convenient as growing CNTs and ZnO nanowires. Some important parameters have been identified to produce high-quality BNNTs on Si substrates. Furthermore, we have identified a series of effective catalysts for patterned growth of BNNTs at desirable or pre-defined locations. This catalytic CVD technique is achieved based on our finding that MgO, Ni or Fe are the good catalysts for the growth of BNNTs. The success of patterned growth not only explains the role of catalysts in the formation of BNNTs, this technique will also become technologically important for future device fabrication of BNNTs. Following our success in controlled growth of BNNTs on substrates, we have discovered the superhydrophobic behavior of these partially vertically aligned BNNTs. Since BNNTs are chemically inert, resistive to oxidation up to ~1000°C, and transparent to UV-visible light, our discovery suggests that BNNTs could be useful as self-cleaning, insulating and protective coatings under rigorous chemical and thermal conditions. We have also established various approaches to functionalize BNNTs with polymeric molecules and carbon coatings. First, we showed that BNNTs can be functionalized by mPEG-DSPE (Polyethylene glycol-1,2-distearoyl-sn-glycero-3-phosphoethanolamine), a bio-compatible polymer that helps disperse and dissolve BNNTs in water solution. Furthermore, well-dispersed BNNTs in water can be cut from its original length of >10µm to(>20hrs). This success is an essential step to implement BNNTs in biomedical applications. On the other hand, we have also succeeded to functionalize BNNTs with various conjugated polymers. This success enables the dispersion of BNNTs in organic solvents instead of water. Our approaches are useful for applications of BNNTs in high-strength composites. In addition, we have also functionalized BNNTs with carbon decoration. This was performed by introducing methane (CH4) gas into the growth process of BNNT. Graphitic carbon coatings can be deposited on the side wall of BNNTs with thicknesses ranging from 2 to 5 nm. This success can modulate the conductivity of pure BNNTs from insulating to weakly electrically conductive. Finally, efforts were devoted to explore the application of the wide bandgap BNNTs in solar-blind deep UV (DUV) photo-detectors. We found that photoelectric current generated by the DUV light was dominated in the microelectrodes of our devices. The contribution of photocurrent from BNNTs is not significant if there is any. Implication from these preliminary experiments and potential future work are discussed.

Role of the cytoplasmic domain in Anabaena sensory rhodopsin photocycling: vectoriality of Schiff base deprotonation.

Light-induced electric signals in intact E. coli cells generated by heterologously expressed full-length and C-terminally truncated versions of Anabaena sensory rhodopsin (ASR) demonstrate that the charge movements within the membrane-embedded part of the molecule are stringently controlled by the cytoplasmic domain. In particular, truncation inverts the direction of proton movement during Schiff base deprotonation from outward to cytoplasmic. Truncation also alters faster charge movements that occur before Schiff base deprotonation. Asp(217) as previously shown by FTIR serves as a proton acceptor in the truncated ASR but not in the full-length version, and its mutation to Asn restores the natural outward direction of proton movement. Introduction of a potential negative charge (Ser(86) to Asp) on the cytoplasmic side favors a cytoplasmic direction of proton release from the Schiff base. In contrast, mutation of the counterion Asp(75) to Glu reverses the photocurrent to the outward direction in the truncated pigment, and in both truncated and full-length versions accelerates Schiff base deprotonation more than 10-fold. The communication between the cytoplasmic domain and the membrane-embedded photoactive site of ASR demonstrated here is likely to derive from the receptor's use of a cytoplasmic protein for signal transduction, as has been suggested previously from binding studies.

A numerical study of Bi-periodic binary diffraction gratings for solar cell applications

In this paper, a numerical study is made of simple bi-periodic binary diffraction gratings for solar cell applications. The gratings consist of hexagonal arrays of elliptical towers and wells etched directly into the solar cell substrate. The gratings are applied to two distinct solar cell technologies: a quantum dot intermediate band solar cell (QD-IBSC) and a crystalline silicon solar cell (SSC). In each case, the expected photocurrent increase due to the presence of the grating is calculated assuming AM1.5D illumination. For each technology, the grating period, well/tower depth and well/tower radii are optimised to maximise the photocurrent. The optimum parameters are presented. Results are presented for QD-IBSCs with a range of quantum dot layers and for SSCs with a range of thicknesses. For the QD-IBSC, it is found that the optimised grating leads to an absorption enhancement above that calculated for an ideally Lambertian scatterer for cells with less than 70 quantum dot layers. In a QD-IBSC with 50 quantum dot layers equipped with the optimum grating, the weak intermediate band to conduction band transition absorbs roughly half the photons in the corresponding sub-range of the AM1.5D spectrum. For the SSC, it is found that the optimised grating leads to an absorption enhancement above that calculated for an ideally Lambertian scatterer for cells with thicknesses of 10 ?m or greater. A 20um thick SSC equipped with the optimised grating leads to an absorption enhancement above that of a 200um thick SSC equipped with a planar back reflector.

Radiative thermal escape in intermediate band solar cells

To achieve high efficiency, the intermediate band (IB) solar cell must generate photocurrent from sub-bandgap photons at a voltage higher than that of a single contributing sub-bandgap photon. To achieve the latter, it is necessary that the IB levels be properly isolated from the valence and conduction bands. We prove that this is not the case for IB cells formed with the confined levels of InAs quantum dots (QDs) in GaAs grown so far due to the strong density of internal thermal photons at the transition energies involved. To counteract this, the QD must be smaller.